Previously, it has been known that austenitic nickel base alloys are of particular value in handling hot concentrated sulfuric acid; for example, of 65% concentration and higher, and provide good, economical and corrosion resistant life in such service. Such alloys are used in pump and valve components that are regularly subjected to hot concentrated sulfuric acid, for example, in the manufacture of acid by the contact process. U.S. Pat. No. 3,758,296 issued to the inventor herein and which is incorporated herein by reference, discloses an alloy of this type.
The previous alloys were quite satisfactory for corrosion service when used as cast pieces; that is, they were effective when cast as pump components, impellers, volutes, and similar parts and had sufficient malleability and ductility that they could be machined as required to achieve proper tolerances and surface finish. However, these alloys did not provide sufficient ductility and hot workability that they could be economically forged, rolled, or drawn to produce highly corrosion resistant bars, wire, sheet, strip, or tubing. The prior alloys typically exhibited sufficient hot ductility to permit casting by commercial processes, but did not exhibit the high degree of hot ductility required for forging, rolling, or drawing without exhibiting structural defects such as breaking and cracking.
I have unexpectedly discovered that it is possible to improve the hot workability and forgeability of my previous alloy to the point so that the new improved alloy is satisfactory and provides highly desirable hot workable and malleable properties so that the alloy can be forged, rolled and drawn to tubes and other structures. I have discovered that by reducing the molybdenum content of the alloy below that previously suggested, the hot workability and malleability of the new alloy is greatly increased. At very low molybdenum contents, high levels of silicon content, preferred for good corrosion resistance and age hardening properties, can be maintained without affecting the improved maleability.
It was unexpected also to discover that alloys having silicon contents over about 2% and low levels of molybdenum content exhibited corrosion rates lower than prior alloys tested. By balancing the content of silicon and molybdenum within the proper range, both effects--that is, increased hot workability and malleability and improved corrosion resistance-- can be achieved. I have found that if the molybdenum content is maintained between about 0%3% by weight, the silicon content is maintained between about 0% and less than about 4% by weight, and where the total sum of the silicon and molybdenum contents is less than about 4% by weight, a highly malleable, hot workable and age hardening hot workable alloy can be produced. This alloy can be hot forged, rolled, or drawn into commercial wrought shapes. I have also found that if the silicon content is maintained on the high side, preferably over about 2% by weight, and the molybdenum content is kept low, preferably under about 2% by weight, superior corrosion resistance is achieved without sacrificing strength or age hardening properties. Alloys having very low silicon contents do have good corrosion resistance and may be desirable for many purposes, but the corrosion resistance in hot concentrated sulfuric acid is generally improved by having the content of silicon higher. Alloys having no molybdenum but a higher level of silicon content exhibit surprisingly high corrosion resistance to this media. Due to the high corrosion resistance of these alloys, they are useful in many environments, for example, as cathodes in anodic protection systems for stainless steel structures. Alloy G582, when used as a cathode, was found to have greater conductivity than Hastalloy C, a conventional material used for this purpose.